Method for leak testing and leak testing apparatus

ABSTRACT

For leak testing closed containers ( 9 ) which are filled with a filling product containing at least one liquid component the container is introduced in a test cavity ( 1 ) which is evacuated at least down to vapour pressure of that liquid component. The pressure in the surrounding of the container ( 9 ) and thus within test cavity ( 1 ) is monitored. Monitoring is performed by a vacuum pressure sensor ( 7 ), whereas lowering pressure surrounding the container ( 9 ) is performed by a vacuum pump ( 5 ). Leakage is detected by monitoring a pressure change in the surrounding of the container which is due to evaporation of liquid emerging from a leak and being evaporated in the low pressure surrounding.

This application is a divisional of U.S. application Ser. No.10/434,111, filed May 9, 2003, now U.S. Pat. No. 6,829,936, which is adivisional of U.S. application Ser. No. 10/193,914, filed Jul. 15, 2002,now U.S. Pat. No. 6,575,016, which is a divisional of U.S. applicationSer. No. 09/944,407, filed Sep. 4, 2001, and now U.S. Pat. No.6,439,033, issued Aug. 27, 2002, which is a divisional of U.S.application Ser. No. 09/785,261, filed Feb. 20, 2001, and now U.S. Pat.No. 6,305,215, issued Oct. 23, 2001, which is a divisional of U.S.application Ser. No. 09/568,288, filed May 10, 2000, now U.S. Pat. No.6,202,477, issued Mar. 20, 2001, which is a divisional of U.S.application Ser. No. 09/073,852, filed May 7, 1998 (claiming priorityunder 35 USC §119 to PCT/IB98/00309, filed Mar. 10, 1998 and claimingpriority to Europe 97108430.6 filed May 26, 1997), now U.S. Pat. No.6,082,184, issued Jul. 4, 2000, which is a continuation-in-partapplication of U.S. application Ser. No. 08/862,993, filed May 27, 1997,now U.S. Pat. No. 5,907,093, issued May 25, 1999.

FIELD OF INVENTION

The present invention is generically directed on a technique for leaktesting closed and filled containers, whereby the filling materialcomprises at least one liquid component.

BACKGROUND

Leak testing techniques according to which closed containers areintroduced in a test cavity which, after having sealingly been closed,is lowered in pressure by a suctioning pump are known. If the containeris not leaking, then once a predetermined pressure has been reached inthe test cavity and thus in the surrounding of a container to be tested,this pressure will be kept substantially constant. If a leak is providedin an area of the container, wherein air is entrapped, a flow of air outof the container will lead to a rise of the surrounding pressure. If aleak is present in the area of the container where filling good isentrapped, the question whether such leak will lead to a significantrise of the surrounding pressure is largely dependent on the kind offilling good as of its viscosity, whether solid particles are present inthe filling good and, obviously, on the largeness of the leak.

Different approaches have become known to accurately detect leaks atsuch product-filled containers, irrespective whether the leak is presentin an air entrapping container area or in a container area covered withfilling good. One such approach which is the topic of the co-pendingEuropean patent application EP-A-0 791 814 and the U.S. patentapplication Ser. No. 08/862,993 proposes to provide an impedancemeasurement, specifically a resistance measurement, just adjacent to theouter wall of the container by means of an electrode arrangement: Assoon as liquid emerges from a leak it will contact a respective pair ofimpedance measuring electrodes and lead to a significant change ofimpedance measured between such electrodes.

Nevertheless, such an approach necessitates considerable additionalexpenditure with respect to provision of the impedance measuringarrangement in each test cavity, especially of a multi-cavity in-lineinspection machine and does not enable detection of very small leaks farbelow of one micron and largely independent from container shape andkind of filling good.

OBJECT OF THE INVENTION

It is a primary object of the present invention to provide a leakagetest method and apparatus, which may be applied to a very large scale ofdifferent containers and of different filling goods, provided at leastone component thereof being liquid.

It is a further object of the present invention to provide such methodand apparatus which are rather inexpensive with respect to electronicand further equipment, and which thus allow for very economic testing.

It is still further an object of the present invention to provide suchmethod and apparatus which have a short measuring cycle and neverthelessa very high measuring accuracy.

SUMMARY OF THE INVENTION

These objects are realised by the testing method for leak testing atleast one closed and filled container, whereby the content of thecontainer comprises at least one liquid component and wherein a pressuredifference is applied across at least a part of the wall of thecontainer which part is to be leak tested and wherein the appliedpressure difference is directed towards the surrounding of the containerand wherein further the pressure in the surrounding of the container ismonitored as a leak indicative signal which is characterised by the factthat the pressure difference is established by lowering the pressure inthe surrounding of the container at least to a value which accords tothe vapour pressure of the at least one liquid component of the fillingproduct of the container to be tested.

The present invention departs from the recognition that if a containeris leaking and liquid is drawn by the lower surrounding pressure to itsoutside this will—at a constant volume of the surrounding—lead toevaporation of the liquid as soon as the surrounding pressure reachesits vapour pressure. This leads to a significant change in surroundingpressure compared with the surrounding pressure which would establish atthe same measuring conditions but with an unleaking container.

Monitoring the pressure in a test cavity containing the container, oncevapour pressure of the possibly leaking liquid is reached reveals asbeing a very accurate technique for leak testing. It has been noted thatby such a technique leak detection of containers with a very largespectrum of filling products may accurately be performed and that leaksat present moment down to 0.02 μm are accurately detectable.

Further, it has been noted that the volume of the test cavity isuncritical, so that by the inventive technique it becomes possible tosimultaneously test batches of containers, thereby accurately detectingif one container of such a container batch is leaking.

As soon as the pressure surrounding a leaking container is lowered withrespect to its interior pressure, some of the liquid is suctioned out ofthe container and as soon as the surrounding pressure reaches vapourpressure it starts to evaporate. As at a constant volume of thesurrounding area of the container evaporation of the liquid leads toincrease of pressure and the pump lowering the surrounding pressure mustnow remove vapour of the liquid too, significant measurements may bedone especially after the surrounding pressure of the container becomeslower than the said vapour pressure. Nevertheless, it is preferred toprovide pumping abilities which may evacuate the surrounding of thecontainer to be tested to a significantly lower value than said vapourpressure, namely by at least two, preferably even by at least threedecades.

As a leak-significant pressure change may be detected as soon as one ofpossibly several liquid components of the filling good starts toevaporate—in the case the content of the container contains more thanone liquid component—it is recommended to select the vapour pressure ofthat component of the several liquid components which is the higher andto lower the pressure of the surrounding of the container at least tothat vapour pressure value.

Although and as well known vapour pressure is a function of temperatureand thus it might be advantageous in some cases e.g. to heat thesurrounding of the container to a predetermined temperature so as tosettle the relevant vapour pressure for a predetermined liquid, theinventive method and apparatus becomes significantly less complex if thetest is performed at room temperature, and thus the vapour pressure tobe at least reached is considered at room temperature, i.e. around 20°C.

Further, a very accurate leak detection becomes possible if thesurrounding pressure of the container is measured at two subsequentpoints in time, whereby we understand under “point” that interval oftime necessary for accurately measuring the prevailing pressure.Although it is absolutely possible to realise leak detection by applyingthe pumping action of the evacuating pump to the surrounding of thecontainer and then by measuring the resulting surrounding absolutepressure after a predetermined time span, the said measuring of thesurrounding pressure at two specific points in time allows to use thefirst value measured as a reference value and then to form thedifference of the second value measured with respect to the referencevalue. There is thereby realised a pressure difference measurementinstead of an absolute pressure measurement. More specifically, thefirst pressure signal which is measured at the first point in time isstored as an electric signal, then, after having measured the secondpressure value, a difference is formed between the first value (stillstored) and the second value.

The PCT patent application No. WO94/05991 with its US counterpart No.U.S. Pat. No. 5,239,859, assigned to the same applicant as the presentinvention, describes a method and apparatus for very accuratelyoffset-compensated pressure difference measurement. In a preferred modeof operating the method according to the present invention as well as ofrealising the inventive apparatus, that pressure difference measuringtechnique and apparatus are used. Therefore, the WO94/05991 or therespective U.S. Pat. No. 5,239,859 are fully incorporated by referencein this present disclosure, although, and as will be seen most importantfeatures are specifically described also in this present application.

Because it is largely uncritical how big the surrounding volume of atest cavity for the container is, with respect to the volume of thecontainer to be tested, the inventive method and apparatus reveals tohave a further significant advantage:

If the wall of the at least one container to be tested withstands thepressure difference between container internal pressure (normallyambient pressure) and lowered surrounding pressure, such a container maysimply be introduced in the test cavity forming the surrounding, largelyirrespective how big such container is with respect to the test cavity.Nevertheless, a highly accurate indication of leakage will inventivelybe gained. Therefore, one and the same test cavity may be used for alarge number of differently sized and different-volume containers. Thisresults in a further advantage in that batches of more than one, even ofa multitude of containers, may be introduced in one test cavity formingthe surrounding and although one single container occupying only a smallpercentage of the overall cavity volume, an accurate leak indicationwill be detected if even only one of the batch-containers is leakinginto the surrounding atmosphere.

A further significant advantage of the present invention is thefollowing:

Sometimes the filled containers are not completely filled, but there issome amount of air entrapped in the closed container. If a leak ispresent in that area of such a container, which is adjacent to entrappedair or gas, by lowering the surrounding pressure, such air will besuctioned through the leak out of the container. With the pressure ofthe entrapped air in the container becoming progressively lower, therewill also start vaporisation of the liquid component within thecontainer and such vapour will also leave through the leak. Both, namelyfirst the air leaving through the leak, then vapour leaving through theleak, will enlarge the surrounding pressure so that a leak in anentrapped air region of the container will lead to a change in thesurrounding pressure, i.e. to rising of said pressure, as if the leakwas in the liquid content covered area of the container wall. Thus, byproperly setting a threshold value for leak detection according to thesmallest still tolerated pressure change in the surrounding, it becomesuncritical whether such leak is present at an air-covered container areaor at a content-covered container area.

If one and the same leak at an air-entrapped area of the container leadsto a smaller pressure change in the surrounding, than the same leakwould generate if situated at a liquid-covered container area, it issuch a pressure change which will govern setting of a threshold value todetect whether a container is leaky or not. If, inversely, one and thesame leak in a liquid-covered area would result in a smaller pressurechange in the surrounding than such leak in an air-contacted wall area,then it is again that smaller pressure change which governs thethreshold setting for detecting leaking/not leaking containers.

If a container under test is largely leaky, lowering of the surroundingpressure should be stopped as soon as such leaking is detected so as toprevent the content of the container to spoil the interior of the testcavity or, generally spoken, the surrounding of the container andpossibly even the pumping arrangement more than absolutely necessary.This is realised either by monitoring whether the pumping action resultsin a predetermined lowering of surrounding pressure or not or one maydetect spreading of content of the container into its surrounding bymeans of an impedance, thereby preferably a DC resistance measurement inthe surrounding of the container just adjacent to the wall of thecontainer which is to be tested. This is realised by providing anelectrode arrangement in said adjacent surrounding and all around atleast that part of the container to be tested. As soon as fillingcontent of the container is suctioned to its outer wall, the electrodearrangement will be bridged by such content, leading abruptly to anindicative impedance change which, after having been detected, is usedto stop further pressure lowering at the surrounding of the container.

This latter technique of rapidly detecting large leaks is appliedespecially to containers where it is necessary to snugly encapsulatethem in the test cavity because their walls would not stand the pressuredifference applied. In such a case the electrode arrangement forimpedance measurement may be incorporated along the inner wall of thetest cavity, which snugly fits with the at least one container. If suchcontainer is to be tested and therefore the test cavity snugly fits itsshape, nevertheless a continuous volume is maintained between the outerwall of the container and the wall of the test cavity for defining thesurrounding of the container by providing a sustaining grid or meshinlay or preferably by roughening the interior wall of the test cavityso that a multitude of micro-embossments of the test cavity wall sustainthe container wall and prevent it from further outward bowing due to theapplied pressure difference. Thereby, the intercommunicating spacebetween such embossments defines for the surrounding space of thecontainer.

Once the container in a test cavity, defining for its surrounding, hasbeen detected as being leaky, it is probable that such test cavity willbe contaminated by some of the container's content. Then, such cavity iscleaned after the leaky container has been removed, be it by evacuationand/or flushing with a flushing gas, preferably nitrogen, be it byheating or by combining these techniques, e.g. by a heated flushing gas.

If the inventive method or apparatus is applied for in-line testingcontainers and thus two or more of the inventive methods and of therespective apparatus are operated in parallel on a set of containers andone of such containers is detected to be leaky, then the respective testcavity defining for its surrounding is not anymore filled with acontainer at the next measuring cycle, but is kept empty, using thatcycle during which the other cavities are in testing condition forcleaning and reconditioning the probably contaminated cavity. Further,it is proposed in some cases to accelerate squeezing-out of liquid, if aleak is present, by mechanically biasing the wall of the containerinwardly, thus rising its interior pressure over atmospheric pressure.

To fulfil the object, the present invention proposes a leak testingapparatus for leak testing at least one closed and filled container,whereby the content of the container comprises at least one liquidcomponent, which comprises at least one sealingly closable test cavityand at least one evacuation pump operationally connected to the testcavity and further at least one pressure sensor operationally connectedto the test cavity, whereby the evacuation pump is selected so as to beable to pump the test cavity to at least vapour pressure of the liquidcomponent of the container content, approx. at room temperature and thepressure sensor is a vacuum pressure sensor, preferably comprising atleast a Pirani sensor stage.

Preferred embodiments of the inventive method and inventive apparatusare disclosed hereinafter. The inventive method and apparatus maypreferably be used for leak testing blisters, vials, medical applicationcontainers, foodstuff or beverage containers, and tanks. Thereby, itmust be pointed out that besides of leak testing of smaller containers,the present invention makes it possible to permanently monitor tightnessof the tanks of huge tank plants, as for gasoline, gases, etc., e.g., ontrain or street transports, thereby generating an alarm signal as soonas a leak is detected.

SHORT DESCRIPTION OF THE FIGURES

The present invention will now additionally be described with the helpof figures showing specific and today preferred examples of realisingthe present invention. Such figures show:

FIG. 1: qualitatively the dependency of vapour pressure from temperatureof a liquid;

FIG. 2: schematically an inventive test apparatus operating according tothe inventive method;

FIG. 3: qualitatively the time course of the pressure of the surroundingof a container to be inventively tested for explaining the inventivemethod and apparatus operation;

FIG. 4: in a functional block diagram a preferred form of realisation ofan inventively operated inventive test apparatus;

FIG. 5: as a functional block diagram a preferred form of realisation ofthe evaluating electronic at an inventive apparatus performing theinventive method;

FIG. 6: schematically batch operation of an inventive apparatus;

FIG. 7: schematically a test cavity for testing flexible wallcontainers;

FIG. 8: in a perspective view one half of a test cavity for inventivelytesting three containers as a batch;

FIG. 9: schematically a double-wall tank directly used to perform theinventive method with an inventive apparatus so as to survey tankleakage;

FIG. 10: schematically a preferred sealing at a test cavity of theinventive apparatus;

FIG. 11 a to 11 c: show the pressure courses on testing cycles, whereatthe containers or medical application blisters are either largely oreven very largely leaking (FIG. 11 a), or have only a small leak (FIG.11 b), or are to be considered unleaky (FIG. 11 c). The tests areperformed with test cavities according to FIG. 8 without impedancemeasurement and thus without electrodes 32, 34.

FIG. 12 a signal flow/functional block diagram of the simplifiedpreferred embodiment of an evaluation unit for operating the inventivemethod at an inventive apparatus;

FIG. 13 in a pressure versus time diagram the statistical variation ofpressure courses measured at unleaky containers or at test cavities voidof any containers,

FIG. 14 in a simplified functional block/signal flow diagram a part ofthe inventive apparatus operating according to preferred mode of theinventive method, thereby forming a dynamic reference value for leaktesting by means of a subsequently updated averaging;

FIG. 15 in a simplified signal versus time diagram qualitatively thepreferred inventive method and accordingly operation of a preferredinventive apparatus, whereby dynamically updated reference values areformed for leak identification;

FIG. 16 a simplified signal flow/functional block diagram showing afurther preferred mode of operation of the inventive method andrespectively of the inventive apparatus, wherein a dynamically updatedaverage signal is formed as the basis for a reference value to becompared with a pressure difference signal evaluated during containertesting;

FIG. 17 in arbitrary unit over the time axis pressure measurements atsubsequently operated test cavities of an inventive apparatus withmultiple cavities to show dynamic update of an average signal, whereonreference values for comparison are based, leading to leakageidentification;

FIG. 18 in a simplified schematic representation, a test cavityaccording to the present invention, which is pivoted during testing;

FIG. 19 the effect of pivoting the test cavity according to FIG. 18 onthe relative location of a leak with respect to filling product;

FIG. 20 in a simplified functional diagram provision of a calibrationstandard leak to calibrate the inventive apparatus as performing theinventive method.

DETAILED DESCRIPTION OF DISCLOSED EMBODIMENTS

In FIG. 1 there is qualitatively shown the course of vapour pressurep_(V)(T) in the pressure versus temperature diagram. At a predeterminedtemperature T_(x) a liquid starts to evaporate when the respectivevapour pressure p_(Vx) is reached. Above the vapour pressure course thematerial is liquid, below the material is gaseous.

According to FIG. 2 an inventive apparatus comprises a test cavity 1with a sealingly closable cover 3. A vacuum pump 5 connected to the testcavity 1 which may be a drag pump or a rotational piston valve pump or adiffusion pump or a turbo vacuum pump as a turbo molecular pump. Thisdepends on the degree of vacuum which shall be established within cavity1. Further, there is provided a vacuum pressure sensor 7 as e.g. aPirani sensor, which measures the pressure prevailing in the test cavity1. At least one closed container 9, which is filled at least to someextent with a filling product containing at least one liquid componentis introduced through opened cover 3 into the test cavity 1 which isthen sealingly closed. By starting operation of vacuum pump 5 thesurrounding of container 9 and thus the intermediate volume V of testcavity and container 9 is lowered.

According to FIG. 3 starting at ambient pressure p_(o) the pressure involume V is lowered at least down to the value P_(v) which accords tothe vapour pressure of the liquid component within the filling good ofthe container 9. It is advisable to select a vacuum pump 5 which enablesto evacuate the test cavity 1 down to a pressure which is at least one,preferably two and even more preferred three decades lower than thevapour pressure p_(V) of the liquid content of the filling product.

The test is preferably performed at room temperature, i.e. at atemperature T of about 20° C. If the liquid content is water then thevapour pressure p_(V) of water at room temperature is about 20 mbar andit then is preferred to provide an evacuation pump 5 which is able toevacuate the test cavity to about 10⁻² mbar.

If the container provided in the test cavity 1 having a relatively rigidwall 11 is not leaky, then qualitatively the pressure in volume V willfollow the course (a) according to FIG. 3 down to the more or lessconstant value of pressure, which may be reached by that type of vacuumpump installed. If, on the other hand, the container 9 is leaky asschematically shown in FIG. 2 e.g. at location 13, then a small amount14 of liquid component of the filling good will be drawn through theleak 13 out of the container 9 and as soon as the pressure prevailing inthe volume V becomes p_(V) starts to evaporate into the volume V. Asqualitatively shown in FIG. 3 this leads to a pressure versus timecourse according to (b), i.e. evaporation of the liquid leads to apressure rise in volume V, counteracting the action of the vacuum pump5. The vacuum pump 5 will have to remove additionally the vapour tofinally achieve a vacuum level according to course (a). If the leak issituated at an area of the container 9 where air is entrapped, as inFIG. 2 at 13′, then evacuation of volume V will first lead to suctioningair out of the container, again counteracting the operation of vacuumpump 5, then the liquid content within container 9 will start toevaporate within the container and vapour will be suctioned out of leak13′. This, too, will lead to a pressure rise in volume V, counteractingthe pressure course which would be followed if just air had to beremoved by vacuum pump S.

By means of the vacuum sensor 7 the course of pressure in the volume Vis monitored. Experiments have shown that largely independent of theamount of volume V in a test cavity a significant difference of pressureaccording to the courses (a) and (b) of FIG. 3 is reached after a timespan τ of a few seconds (one to three seconds) and at a leak smallerthan 1 micron (0.02 μm), the pressure difference between a leaky and anunleaky container being of about one pressure decade. Measurements wereperformed with water as liquid content.

Although it is absolutely possible to measure the absolute pressure involume V, e.g. after the time span τ to detect leakage of the containera pressure difference measurement is preferred, as will first beexplained with the help of FIG. 4.

Back to FIG. 2 the pressure sensor 7 is operationally connected to anevaluating unit 15, whereat especially leak indicative threshold valuesare preset, as schematically shown by means of a presetting unit 17. Theoutput of the evaluation unit 15 is a two-state signal indicating leakyor not leaky.

According to FIG. 4 the output of the vacuum sensor 7 is input to astorage unit 19, controlled by a timing control signal s₁, asschematically shown via switch S. According to FIG. 3 this is performedat a first point in time t₁. At a second point in time, according toFIG. 3 t ₂, the output of the storage unit 19 and the output of thesensor 7 are connected to respective inputs of a difference forming unit21, which generates an output signal which accords with the pressuredifference Δp of FIG. 3.

A further, most preferred realisation of the evaluation electronic isshown in FIG. 5. The output signal of sensor 7 is input to a conversionunit 121, which comprises, as an input stage, an analogue to digitalconverter 121 a, followed by a digital to analogue converter 121 b. Theoutput of the converter stage 121 is fed to a difference amplifier unit123, which additionally receives directly the output signal from sensor7. The output of the difference amplifier unit 123, according to thedifference unit 21 of FIG. 4, acts on a further amplifier unit 125, theoutput of which being superimposed at 128 to its input via storage unit127. The input of the storage unit 127 is fed from the output of unit125. A timer unit 129 time controls the arrangement. For storing a firstpressure value from sensor 7, according to FIG. 3 at time t1, the timerunit 129 enables a conversion cycle at unit 121, so that a reconvertedanalogue output signal el_(o) appears at the output. Simultaneously, thesubstantially same signal from sensor 7 is applied as signal el to thesecond input of unit 123. Thus, at the output of unit 125, a zero signalshould appear. Nevertheless, in general a zero-offset signal will appearat the output of unit 125, which signal is stored in the storing unit127, enabled by the timing unit 129. At time t2 no conversion istriggered at the unit 121, so that there appears at the input ofamplifier 123 directly from sensor 7 the pressure value prevailing at t₂and, from stage 121, the stored pressure value which was prevailing att₁. Further, the zero offset signal which was stored in unit 127 issuperimposed as a offset-compensating signal so that the resultingsignal at the output of amplifier unit 125 is zero-offset compensated.

This allows a very accurate measurement of pressure difference Δpaccording to FIG. 3.

If the container under test has a large leak, then, and according toFIG. 3 course (c) the pressure prevailing in the volume V of the testcavity 1 will have just from the beginning of operating the vacuum pump5 a different course. This may easily be detected, e.g. by comparing ata previous point in time t₀ the output signal of sensor 7 with apredetermined threshold value (not shown), and if such threshold valueis not reached by the actual pressure, the effect of the vacuum pump 5on test cavity 1 is disabled. This to avoid that, with a larger leak, ahuge amount of content of the container is suctioned into the testcavity and contaminates that cavity.

As was mentioned, the proposed method accurately functions largelyindependently from the volume V between test cavity 1 and the at leastone container to be tested. This allows, according to FIG. 6, tosimultaneously test batches 9′ of containers 9, thereby maintainingaccuracy of detecting whether one or more than one of the containers 9leak. Further, the fact that detection accuracy is not critical withrespect to difference volume V leads to the possibility of providing onetest cavity 1 for a multitude of differently shaped and different-volumecontainers 9 to be tested therein.

If the wall of a container to be tested may not mechanically withstandthe pressure loading of approx. 1 bar, then, and as schematically shownin FIG. 7, a test cavity 1′ with cover 3′ is provided which snugly fitswith the shape of the container 9. Thereby, protrusions 20, asschematically shown in FIG. 7, prevent that by effect of the evacuationthe walls of the container are firmly suctioned on to the inner wall ofthe test cavity and thus make sure that there remains a volume V betweencontainer and test cavity wall for being evacuated according to theinvention. Such protrusions 20 may be realised by a mesh or grid inlayor, and preferably, by mechanically roughening the inner wall of thecavity, so that micro-embossments sustain the wall of the container,thereby leaving a continuous interspace as volume V.

As shown in dashed line in FIG. 7 it might further be advantageous, e.g.when closing the cover 3 or 3′ of the cavity, to mechanically bias apart of the container's wall inwardly, thereby increasing the innerpressure of the container 9 and additionally pressing liquid componentof the filling product out of a leak if such a leak is existent.

According to FIG. 9 the method and apparatus according to the presentinvention may be used to monitor huge tanks with respect to leakage. InFIG. 9 there is shown a tank with double-wall, namely with an inner wall23 and an outer wall 25. Testing tightness of both these walls isperformed by using the intermediate volume of the two walls, as volume Vaccording to FIG. 2. Such a technique may be applied e.g. for tanks onroad or rail vehicles or for huge stationary tank plants, e.g. forgasoline.

In FIG. 8 there is shown one half 1 a of a test cavity 1 for applyingthe inventive method in an inventive apparatus on three containers at 29as on small plastic containers for medical appliance. The containers mayhave flexible walls as the test cavity 1 snugly fits their shape. Thereis further shown another technique to rapidly detect whether one of thecontainers has a large leak. There are provided impedance measurementelectrodes 32 and 34 integrated in the wall of the cavity 1 and mutuallyelectrically isolated. They are connected to an impedance or,preferably, resistance measuring unit 35. If by applying a vacuum to thetest cavity, preferably with a roughened interior wall, liquid fillingcontent is suctioned to the outside of the container wall, this isquickly detected by an abrupt change of impedance measured between theelectrodes 32 and 34. The output of the impedance measuring unit 35disables (not shown) further evacuation of the test cavity 1.

Once a test cavity has been spoiled by outpouring filling good of aleaking container it is cleaned, either by cleaning evacuation and/orpouring with a gas, preferably with nitrogen, and/or by heating. In FIG.8 there is shown a feeding line for a flushing or cleaning gas,controllably fed from a gas tank 37 to a contaminated test cavity 1,which gas preferably is nitrogen.

Two cavity halves, 1 a according to FIG. 8 are sealingly put one uponthe other to complete a test cavity 1 according to FIG. 2.

If in-line testing of containers shall be performed, for which thepresent invention is especially suited due to its short measuring cycle,more than one, namely a set of several test cavities is provided, e.g.on a carousel, which are automatically loaded with containers to betested (not shown) from a conveyor and which perform simultaneously thedescribed testing technique. If one of the containers tested in suchcavity is detected to be leaky, then the respective cavity is notreloaded with a further container afterwards, but this cavity ismaintained empty during the measuring cycle on a next set of containers.Meanwhile, the cavity kept unloaded is cleaned, as was described, eitherby evacuation and/or gas flushing and/or heating.

Obviously, there must be realised a good vacuum-tight sealing between acover 3 or 3′ of the test cavity and the main body of the test cavity 1or between the two halves 1 a of test cavity according to FIG. 8. Thisis realised preferably by providing at least a pair of parallel seals 28as of concentric O seals and by separately pumping an intermediate space29 between such seals, as shown in FIG. 10. If the container to betested contains a filling product with more than one specific liquidcomponent, the vapour pressure of that component is selected for leakdetection which has the highest vapour pressure, i.e. which componentstarts to evaporate at relatively highest pressure. Thereby, viscosityhas to be considered too, i.e. a component is to be selected fordefining the vapour pressure, which component is liquid enough topenetrate smallest leaks. By evacuating the test cavity down to apressure which is significantly lower than the vapour pressure of anyliquid component it becomes uncritical which vapour pressure value is tobe considered.

Pressure versus time courses as measured according to the inventivemethod and with an inventive apparatus, both in preferred mode, areshown for containers with large leaks in FIG. 11 a, for small leaks inFIG. 11 b and for unleaky containers in FIG. 11 c.

These figures shall be discussed in connection with FIG. 12, which showsa preferred monitoring and control unit according to units 15, 17 ofFIG. 2.

According to FIG. 11 a the timing unit 201 of FIG. 12 initiates at timet₁₀ evacuation of a test cavity 103 by means of the pumping arrangement105. This is shown in FIG. 12 by the evacuation start signal EVST/t₁₀.

After a fixed predetermined amount of time ΔT of e.g. 0.75 sec. theoutput signal of the pressure sensor within test cavity 103 (not shownin FIG. 12), A_(s), becomes compared with a first reference signalpreset at a presetting source 107, RFVGL. To this target, comparatorunit 109 is enabled by timer unit 201 at t₁₀+ΔT.

If after time span ΔT the actual monitored pressure according toelectric signal A_(s) of FIG. 12 has not reached the value of RFVGLaccording to course I of FIG. 11 a, this means that a very large leakVGL is present. This is detected at comparator 109 generating the outputsignal A₁₀₉. If according to the characteristics shown in the block 109of FIG. 12 the output signal of this comparator unit 109 enabled att₁₁=t₁₀+ΔT is e.g. still at a high level indicating presence of a VGL,this is output at the VGL output. If the pressure prevailing in thesurrounding of the container 103 under test, i.e. in the test cavity,has reached and crossed reference level RFVGL according to course II ofFIG. 11 a, the VGL output signal is not generated.

As will be explained later, occurrence of the VGL signal preferablystops the evacuation cycle because contamination of the vacuum pump 105may have occurred or might occur due to the very large leak of thecontainer under test.

As shown by the course II of FIG. 11 a as VGL does not occur evacuationcontinues up to a further moment of time t₁₃. At the time t₁₃ the timerunit 201 disables pumping arrangement 105 and disconnects as by a valve106 the pumping arrangement from chamber 103. Further, timer unit 201enables comparator unit 111, to which a further reference value RFGL isled, generated by a reference signal source 113. If at t₁₃ the pressureprevailing in the surrounding of the test cavity has not reached RFGLthen comparator unit 111 generates an output signal GL indicating thatthe container under test has a large leak. Here again, and as will befurther explained later on, some reactions are taken with respect tofurther operation of the testing system.

If either the signals VGL or GL are initiated by the respectivecomparators 109, 111, the timer unit 201 is principally reset becausethe testing has been completed and the quality of the instantaneouslytested container established has been identified. This is schematicallyshown in FIG. 12 by the signal RS₂₀₁. If not reset, shortly after t₁₃the value A₅ (t₁₃) of the pressure prevailing in the surrounding of thecontainer is stored in a holding or storing unit 117. The output of theholding or storing unit 117 is led to one input of a difference formingunit 119, whereas the second input of this unit 119 is connected to theoutput A₅ of the pressure sensor monitoring the pressure in thesurrounding of the container under test. After a presettable test cycletime T_(T) starting at t₁₃, as schematically shown by unit 121 of FIG.12, the pressure difference DP at the output of the unit 119 isevaluated, as represented in FIG. 12 by switching unit 123. Thispressure difference DP is fed to a further comparator unit 125 enabledat the lapse of testing time T_(T). By means of a further referencevalue source 127 the reference value DPREF is fed to the comparator unit125. As will be explained later, the value of DPREF may controllably bevaried in time and/or a reference value Φ_(R) to which DPREF is referredto may also controllably be varied in time.

If DP at time t₁₃+T_(T) is larger than the reference value DPREF, then asignal FL is generated at unit 125, indicating presence of a fine leakFL in the container under test. This according to the situation as shownin FIG. 11 b. If DP does not reach DPREF, then the container isconsidered unleaky as none of the signals VGL, GL and FL have beengenerated. This according to FIG. 11 c.

If the VGL signal is generated according to FIG. 12, the evacuation pump105 is immediately disconnected from any testing chamber 103 it isconnected to, be it a single chamber or be it in an in-line processingwhere one pump 105 is parallel connected to a multitude of testingchambers 103, from all such chambers. This because at a very large leakthe vacuum pump 105 could have been contaminated by leaking content ofthe container. It thereby is absolutely possible to provide for such acase a redundant pumping arrangement which may be connected to the oneor the more than one testing chambers to continue testing, whereas thepossibly contaminated first pumping arrangement is reconditioned.

In a multiple chamber in-line testing system, as e.g. in a carouseltesting plant with a multitude of testing chambers, occurrence of thesignal GL indicating a large leak and possibly also the occurrence ofthe signal FL indicating for a fine leak leads preferably to disablingor “bypassing” that chamber with the leaky container from further beingsupplied with containers to be tested, whereas the other chambers arestill operating and performing tests on newly supplied containers. Thisbypass of a testing chamber, whereat a container has been identified asheavily or even slightly leaking, is performed so as not to influencefurther testing results at that chamber which wouldn't thus berepresentative anymore due to content of the leaky container havingpossibly contaminated that chamber.

This bypassed chamber is reconditioned during further testing cycles atthe other chambers.

Reconditioning may be done by heating that chamber, flushing it by aliquid and/or a gas, especially by a heater gas. Whether or not thatchamber has been properly reconditioned is checked by having it testedas if it was filled with a container to be tested. Thereby, thecondition of proper reconditioning is indicated if DP according to FIG.12 at that empty chamber does e.g. not reach DPREF or an appropriatelyset “Empty Chamber DPREF”-value (ECDP-REF).

Such ECDP-REV may be provided by measuring DP_(e) at the clean, emptytest chambers and by storing these measuring values DP_(e) as respectivereference values for testing the chambers on proper reconditioning.

When looking to the FIG. 11 a to 11 b, it may by recognised that settingthe reference value RFGL and especially setting of the referencepressure difference value DPREF may be very critical and may largelyinfluence accuracy of the system. Thereby, influences as surroundingtemperature, moisture of ambient air, slight contamination of pump etc.,may influence the prevailing pressure course and lead to false resultsif these critical reference levels and especially DPREF are set forutmost accuracy.

In FIG. 13 there is quantitatively shown the pressure course accordingto the courses of FIG. 11 a to 11 c, but measured at test cavities voidof containers. At t₁₃ there occur statistically distributed slightlydifferent pressure values. Thus, before beginning testing of containersat a multiple test cavity plant, the unfilled, tightly closed testcavities are tested according to FIG. 13 to establish an average(RFGL)_(m). The value of RFGL as used at the comparator 111 of FIG. 12or as used according to the FIGS. 11 a to 11 c is found in that anoffset value ΔRFGL is added to (RFGL)_(m). It must be pointed out thatambient parameters as temperature, humidity of ambient air etc. may beconsidered constant during the calibrating cycle performed at the emptyand conditioned test cavities and leading to the measuring resultsaccording to FIG. 13. Nevertheless, during ongoing time as duringon-line testing, these disturbing parameters may slowly change and mayvary (RFGL)_(m).

Every time during multiple or in-line testing, be it subsequently with asingle test cavity or consecutively with a multitude or at least morethan one test cavity, at the respective time t₁₃, up to which therespective container has been identified as not heavily leaky, theactual output signal of the pressure sensor is entered into an averagingunit 113, wherein the last m values of actual pressure of not heavilyleaky containers are averaged. The output average result signal accordswith (RFGL)_(m) of FIG. 13, but varies in time, e.g. due to varyingambient parameters. To the output average result {overscore (A5)} andaccording to FIG. 13 the offset ΔRFGL is added, the result of thataddition is a dynamically varying reference value RFGL, which is appliedto comparator unit 111 of FIG. 12. This dynamically varying referencevalue RFGL is shown in FIG. 15, starting from an initial setting, ase.g. found as was explained with the help measurements at empty testcavities 103.

As may clearly be seen now from FIG. 15, the average pressure value{overscore (A5)} (t₁₃) is now the basis for also referring DPREF to.Therefore, and as shown in FIG. 12, the difference pressure referencevalue DPREF is not referred to an absolute static value as φ_(R), but isreferred to {overscore (A5)}.

An even further improvement of accuracy is reached as will now bedescribed, which may be realised separately or additionally to realisinga dynamic RFGL and based thereon a dynamic upper limit of DPREF. Therebyand according to FIG. 16 at the end of the time span T_(T) the actualpressure difference DP is led to an averaging unit 135 whenever theoutput signal FL indicates that the container under test is unleaky. Theoutput signal of unit 135 which accords to an average pressuredifference signal {overscore (DP)} averaged over the last m test cycles,is offset by an amount ΔDP, the result thereof being used as DPREFsignal applied at unit 127 of FIG. 12.

Looking back on FIG. 15, whereat, as discussed before, a constant DPREFsignal was applied the technique of averaging DP results, asschematically shown with a course (DPREF)_(t), in a dynamically varyingcheck value DPREF, varying according to variations of disturbingparameters, influencing such pressure difference.

It is clear that provision of a dynamically varying (DPREF)_(t) signalaccording to that representation in FIG. 15 could be realised withoutproviding a dynamically varying base value {overscore (A5)}, inreferring (DPREF)_(t) to a stable, constant value φ_(R), as shown inFIG. 12 in dashed representation instead of referring to a dynamicallyvarying {overscore (A5)} value.

It is evident that preferably the evaluations of the output signal A₅ ofthe one or more than one test cavities is performed digitally, i.e.after analogue to digital conversion of the output signal of therespective sensor or sensors.

In FIG. 17 there is shown over the time axis and in arbitrary units theactual pressure difference values DP measured successively at amultitude of test cavities of an in-line testing plant. According toFIG. 16 the calculated average pressure difference {overscore (DP)} isshown and the offset ADP finally leading to (DPREF)_(t) according toFIG. 15 or 16. As may clearly be seen, the average {overscore (DP)} andthus (DPREF)_(t) vary in time and along successive testing, wherebypressure difference values as at A, which are higher than theinstantaneously prevailing (DPREF)_(t), are disregarded with respect toinfluencing the averaged {overscore (DP)}, as such measurements are dueto leaky containers according to FIG. 11 b.

Further, whenever the test of a container within a specific test cavityresults in a leak-indication for a predetermined number of subsequenttests, as e.g. three times subsequently, such test cavity is alsobypassed for further testing and is considered as contaminated or asleaky itself, thus being reconditioned. Such a test cavity is likely tohave been contaminated during succeeding testings at leaky containers oris likely not to be tight, which will be recognised duringreconditioning and testing on proper reconditioning too, as wasdescribed above.

Further, and as was already mentioned, for some containers to be testedand especially for some filling products it is advisable to heat thetest cavities to a predetermined temperature which is preferablycontrolled at each test cavity, e.g. by a negative feedback temperaturecontrol. Thereby, the temperature-dependent evaporation pressure of thefilling product is set within a predetermined pressure range. Suchheating is thereby preferably accomplished in a pre-heating cycle beforethe actual testing cycle according the FIGS. 11 a to 11 c is performed.

As was mentioned above, a leak in a container will be identifiedirrespective of the fact whether such leak is in an area of container'swall exposed to entrapped air within the container or to the fillingproduct. Nevertheless, for some filling goods as e.g. with particulatecontent in liquid, there might occur differences with respect to time arespective pressure difference develops in the surrounding of thecontainer under test.

Therefore, and as schematically shown in FIG. 18, it may be advisable insome cases to provide the one or the several test cavities 103 for thecontainer to be tested 9 to be movable. This is e.g. accomplished bymounting the test cavities 103 pivotable with respect to a pivot axis Aand driven via a rotational axis 140. Thereby, leads to and from thepressure sensor within such test cavity, to and from a heatingarrangement at such a test cavity etc. may be led through the drivingaxis 140. The cavity 1, 103 is preferably not rotated, but is rotatablyoscillated as shown by ±φ in FIG. 18. By this technique, and asschematically shown in FIG. 19, a leak L is moved into air and intoliquid contact, so that testing will consider vaporising of liquidcontent whenever it occurs, be it in the position according to FIG. 19 aor in the position according to FIG. 19 b.

Proper functioning of the testing apparatus and calibration of theevaluation unit, be it a one-chamber tester or a at multiple-chambertesting plant as for in-line testing, is further preferably accomplishedwith the help of a standard leakage arrangement which is preferablymounted on the test plant, so that recalibration and/or overall testingof the plant may be accomplished whenever desired. The arrangement ofsuch a standard or calibration leak arrangement is shown in FIG. 20.

According to FIG. 20 there is provided in the line from a test cavity,as 103 according to FIG. 12, to the vacuum pump 105 a needle valve 142,which is adjustable but which preferably is preset not variable by theuser of the plant on a predetermined leakage value. Via the needle valve142, the line to the vacuum pump 105 is connected to a liquid reservoir144, which preferably is filled with distilled water. Via a pressurisingline and valve 146 the reservoir 144 may be adjustably pressurised. Theneedle valve is set to such a value that no distilled water of reservoir144 will penetrate into the connection line of chamber 103 to vacuumpump 105, but only vapour. Nevertheless, by adjusting pressurisation ofthe water within reservoir 144 via line and valve 146 a leak ofdifferent and varying extent may be simulated without liquid penetratingand spoiling chamber and/or connection line and/or vacuum pump. For aplant with a multitude of testing cavities such a calibrationarrangement with needle valve 142 may centrally be provided andconnected in parallel to all chambers 103, as in such a plant preferablythere is provided one central pumping arrangement 105 acting in parallelon all the chambers or cavities provided. Alternatively such acalibration arrangement may be provided separately for each of thechambers 103 provided.

It has been recognised that by applying the described technique of leaktesting by lowering the surrounding pressure of a container under testbelow vapour pressure of a liquid component of its content, it is mostlynot necessary to additionally provide resistance measurements, as wasexplained with the help of FIG. 8, so that, at the respective testchambers, the electrode arrangements and measurement units may beomitted, which significantly reduces costs for the overall plant and itscomplexity. The invention is especially suited for testing vials orblisters, especially for medical appliances, in-line with theirproduction by checking every singly vial or blister. If and asschematically shown in FIG. 6 a multitude of containers 9 aremechanically linked together to form a set of such of containers,clearly such a set is considered as one container with respect to leaktesting.

With the inventive method and apparatus as for blisters the entiretesting cycle, i.e. from t₁₀ to the end of T_(T) according to the FIG.11 is performed in less than 2 sec. This leads at an in-line plant witha multitude of test cavities, e.g. with 24, e.g. arranged on a carousel,to a very high throughput.

1. An apparatus for manufacturing unleaky containers comprising: meansfor subjecting at least one container to test conditions and therebymonitoring a signal which is dependent on leak condition of saidcontainer; means for generating at least one signal derived from saidsignal monitored; means for further monitoring whether said signalderived from said signal monitored is or is not within a dynamic signalrange; means for identifying said one container as unleaky in dependencyof a result of said further monitoring; and means for enabling a furthersignal derived from said signal monitored to update said dynamic signalrange if said result of said identifying reveals that said container isunleaky.
 2. The method of claim 1, wherein said means for subjecting atleast one container to test conditions simultaneously subjects a batchof containers to test conditions.
 3. The apparatus of claim 1, furthercomprising means for forming a difference signal of said signalmonitored and at least one reference signal and updating a furtherdynamic signal range by said difference signal, said dynamic signalrange being dependent from said further dynamic signal range.
 4. Theapparatus of claim 3, including means for forming said dynamic signalrange by a result signal of adding said further dynamic signal range toa constant signal value.
 5. The apparatus of claim 3, further comprisingmeans for establishing said at least one reference signal as a dynamicreference signal.
 6. The apparatus of claim 5, wherein said means forestablishing establishes said dynamic reference signal by averaging asignal dependent on said signal monitored if the actual container undertest is identified as unleaky with such signals dependent on containerspreviously identified as unleaky.
 7. The apparatus of claim 3, includingmeans for establishing said reference signal by sampling a signaldependent on said signal monitored at a first point in time and whereinsaid means for generating derives said signal derived from said signaldependent sampled at a second subsequent point in time.
 8. The method ofclaim 7, wherein said means for generating derives said signal derivedfrom said signal dependent from a difference signal from said signaldependent and said reference signal.
 9. The method of claim 7, whereinsaid means for establishing said reference signal stores said signaldependent sampled at said first point in time at least up to said secondpoint in time as an electrical signal.
 10. The method of claim 9,wherein means for establishing includes an analog to digital converterwhich is enabled for conversion at said first point in time for storingsaid signal dependent sampled at said first point in time.
 11. Anapparatus for manufacturing closed, unleaky containers comprising: meansfor manufacturing closed containers; means for subjecting at least oneof said containers to test conditions and thereby monitoring a signal,which is dependent on leak condition of said container; means forgenerating at least one signal derived from said signal monitored; meansfor comparing said signal derived with a dynamic threshold value foridentifying leak condition of said container; means for enabling afurther signal derived from said signal monitored to be averaged withsimilar further signals of other containers previously tested asunleaky, if said leak condition identifying of said container now undertest reveals an unleaky container; means for deriving said thresholdvalue from the result of said averaging; means for identifying said onecontainer as unleaky in dependency of a result of said comparing.